Alkylation of aromatic hydrocarbons utilizing crystalline aluminosilicate catalysts has heretofore been described. U.S. Pat. No. 2,904,607 to Mattox refers to alkylation of aromatic hydrocarbons with an olefin in the presence of a crystalline metallic aluminosilicate having uniform pore openings of about 6 to 15 Angstrom units. U.S. Pat. No. 3,251,897 to Wise describes alkylation of aromatic hydrocarbons in the presence of X- or Y-type crystalline aluminosilicate zeolites, specifically such type zeolites wherein the cation is rare earth and/or hydrogen. U.S. Pat. No. 3,751,504 to Keown et al. and U.S. Pat. No. 3,751,506 to Burress describe vapor phase alkylation of aromatic hydrocarbons with olefins, e.g. benzene with ethylene, in the presence of a ZSM-5 type zeolite catalyst.
The alkylation of toluene with methanol in the presence of a cation exchanged zeolite Y has been described by Yashima et al. in the Journal of Catalysis 16, 273-280 (1970). These workers reported selective production of para-xylene over the approximate temperature range of 200.degree. to 275.degree. C., with the maximum yield of para-xylene in the mixture of xylenes, i.e. about 50 percent of the xylene product mixture, being observed at 225.degree. C. Higher temperatures were reported to result in an increase in the yield of meta-xylene and a decrease in production of para and ortho-xylenes.
While the above-noted prior art is considered of interest in connection with the subject matter of the present invention, the methylation process described herein utilizing a finely divided solid catalyst which has a controlled particle size distribution and which is fluidized in a turbulent sub-transport regime to achieve unexpectedly high selectivity for the production of para-xylene and a high efficiency for the utilization of the methanol alkylating agent has not, insofar as is known, been heretofore described.
Of the xylene isomers, e.g. ortho-, meta- and para-xylene, the latter is of particular value being useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers such as Dacron brand fiber available from the DuPont de Nemours Company of Wilmington, Del. Mixtures of xylene isomers either alone or in further admixture with ethylbenzene, generally containing a concentration of about 24 weight percent para-xylene in the equilibrium mixture, have been previously separated by expensive superfraction and multistage refrigeration steps. Such process, as will be realized, has involved high operation costs and has a limited yield.
U.S. Pat. Nos. 3,965,209 to Butter et al. and 4,067,920 to Kaeding teach processes for producing para-xylene in high yield by reaction of toluene with methanol. The reaction is exothermic, generating about 16.5 kcal/mol (280 Btu/lb) of para-xylene produced. The use of steam co-feed (U.S. Pat. Nos. 4,935,574 and 4,843,057 to D'Amore et al.) or multiple reaction beds with interstage methanol addition (U.S. Pat. No. 4,761,513 to Steacy) have been disclosed as means to quench the reaction exotherm and to improve operability of a fixed-bed process.
Under typical reaction conditions the process is also prone to rapid catalyst deactivation. The use of hydrogen co-feed (U.S. Pat. No. 4,444,989 to Herkes) has been disclosed to reduce coking. Kaeding (U.S. Pat. Nos. 4,067,920 and 4,158,024 to Kaeding) discloses the use of a fluidized catalyst zone but with no teaching of controlling the fluidization regime and the catalyst particle size distribution to enhance para-xylene selectivity and methanol utilization efficiency.